The invention relates to a self-drilling screw produced from an austenitic or other stainless base material, comprising a shank which has a cutting or forming boring tip or thread crest arranged on a shank end and a hard edge layer formed on the surface of the shank in the region of the boring tip or thread crest and at least in the region of a thread connecting thereto.
DE 10 2012 009 400 B4, which relates to a patent also by the applicant, discloses a self-drilling screw formed in one piece. This known self-drilling screw has a simpler design than a two-part self-drilling screw in which the two parts are welded together, one of these two parts comprising the boring tip and the thread-forming region and, by way of example, consisting of carbon steel which has, in a final step, been hardened by induction or flame hardening.
This known screw is only ever used once. A hard coating which is galvanically applied to the boring tip or thread crest and comprises a transition metal layer or chromium layer has been found to be stable for the single drilling process required for this purpose. Simple coating such as chromating or hard chromating or, more generally, galvanically applying the hard coating as a layer that contains a transition metal, is sufficient at least with suitable additives to be able to use the known self-drilling screw at least once. In this case, contrary to the opinion of a person skilled in the art, hard coatings of this type can be achieved without the eggshell effect occurring. The self-drilling screw according to the aforementioned document achieves, owing to the coating applied, the required drilling capacity and the required thread formation.
A self-drilling screw of the type mentioned at the outset is known from U.S. Pat. No. 5,419,948 A. In order to harden said known self-drilling screw, it is chemically reacted, i.e. nitrated, with nitrogen trifluoride on its surface. As a result of the nitration, the known screw, which is stainless per se, produces an ultra-hard surface layer which is susceptible to rust and has to be removed again after the nitration, specifically in the region of the screw head.
U.S. Pat. No. 4,802,807 A discloses a self-drilling screw for which corrosion resistance by galvanising, i.e. by galvanically applied zinc, is achieved. In addition, for this known self-drilling screw, an aluminium protective layer is produced on the zinc protective layer. The aluminium protective layer is applied to the galvanised screw by means of ion vapour precipitation technology. The composite protective layer thus produced is intended not only to improve corrosion resistance but also to reduce the torque when screwing in the self-drilling screw. Finally, a topcoat is applied to the self-drilling screw in order to prevent the aluminium protective coating from oxidising.